Two of the main causes of turbo machinery failure are blade vibration and disk cracks. To achieve the high levels of performance required of modern aircraft, blade and disk designs attempt to achieve high operating stress levels while at the same time minimising size and weight. The complexity of blade shapes, corrosive environments, high speed operation, and severe thermal and dynamic loads all contribute to blade degradation over time. In addition to wear out issues, Foreign Object Debris (FOD) can cause immediate blade damage when particles of significant mass are ingested by the engine. The impact of a damaging item of FOD on a highly stressed compressor blade may not always produce visible damage, but the impact will have altered the stress within the blade. This can drastically alter the life of the blade and possibly instigate early fatigue failure, often with catastrophic effect.
Blade and disk problems are very difficult to detect with typical on-board sensors, such as shaft proximity probes and case mounted vibration sensors, since these problems do not translate to measurable disturbances. Due to the high cost of in-service failures of aircraft engine components and the difficulty of installing on-engine sensors capable of detecting blade problems, visual inspections of aircraft components are required at a conservative interval as a preventative measure. Maintenance inspections are costly due to the manpower and equipment required to perform the inspections and also in lost revenue when assets are taken out of service.
Furthermore, blade tip clearance control is one of the main parameters governing turbine efficiency. Large clearances lead to rapid efficiency drop off, low clearances lead to the risk of rubbing, or even catastrophic failure. But the prospect of percentage point increases in fuel efficiency attainable by blade clearance control systems makes them highly attractive. Tip clearance monitors have been successfully used by gas turbine manufacturers to verify blade tip to frame clearances in prototype engines for many years, while in recent years active clearance control mechanisms have been developed. Since tip clearance varies with component temperatures, achieving full control and maximising the benefit in fuel efficiency requires continuous real time monitoring, and the use of that information as one signal in the adaptive control loop that controls running turbines.
Tip clearance and other blade monitoring processes require data returned from the blade tip itself. Such data has typically been obtained using a highly focussed, narrowband signal (typically less than 10 degrees wide), directed at the blade tip, for accurately collecting data from each blade tip as it passes the sensor. One of the problems with this type of sensing is that the returned data is of little utility in other, more general, engine health monitoring processes. Thus, such monitoring processes tend to require a dedicated sensor system which makes their integration into a more general engine health monitoring system difficult and costly to achieve.